1. Field of the Invention
The present invention relates to a rubber composition to be suitably used for automobile tires, in particular for a tread material thereof, and also relates to a process for producing the same. More particularly, the invention relates to a rubber composition capable of lowering rolling resistance of the tires and to a process for producing the same.
2. Description of Background
In the recent history of the automotive industry, demands for economy, such as saving of natural resources and saving of energy, have been important tasks to be undertaken in addition to demands for durability and safety, and the reduction of fuel consumption has been required for automobiles in order to meet such demands. Thus, the automobile tires of today are required to have even lower rolling resistance, which lower rolling resistance corresponds to a reduction in fuel consumption.
The rolling resistance of automobile tires is caused by energy scattering when the rubber tires undergo periodic deformation at relatively low frequency during driving, and it can be lowered by decreasing the energy loss (hysteresis loss) during driving. It is known that the rolling resistance corresponds to tan .delta. (loss factor to be an index for the hysteresis loss) at around 60.degree. C. by converting the deformation frequency to the temperature in accordance with the Williams-Landel-Ferry equation of a temperature conversion rule. Therefore, rubber materials having a small tan .delta. at around 60.degree. C. have been demanded in order to lower the rolling resistance.
For lowering the rolling resistance, there have heretofore been known such methods as improvement in microstructure or molecular weight distribution of the rubber, improvement in compounding formulation of organic rubber chemicals or reinforcing agents, and addition of modifiers. Among them, the addition of modifiers is drawing public attention, since it can lower the rolling resistance more easily than other methods and can also be applied to a natural rubber.
As an example of such a modifier, EP-A-253365 (the entire disclosure of which is herein incorporated by reference) has proposed a dinitrodiamine compound represented by the following formula (I): ##STR2## wherein X is a divalent aliphatic, alicyclic or aromatic group which may contain halogen or oxygen in the group; R.sup.1 is hydrogen or an aliphatic, alicyclic or aromatic group, provided that two nitrogen atoms linking through X may further link through R.sup.1 when both X and R.sup.1 are the aliphatic groups; and R.sup.2 and R.sup.3 independently of one another are each hydrogen or an alkyl of 1 to 12 carbon atoms, provided that R.sup.2 and R.sup.3 may conjointly form a ring.
This dinitorodiamine compound can produce excellent rubber compositions which have lower rolling resistance when the compound is incorporated into natural or isoprene rubbers.
Therefore, this dinitrodiamine compound has been intended to be applied to rubber tire materials, and in particular, it has exhibited sufficient characteristics in conjunction with such natural rubbers or isoprene rubbers. However, its effects for synthetic rubbers, such as styrene/butadiene copolymer rubbers, are not necessarily satisfactory in satisfying the demand for the reduction of fuel consumption, which has become more severe in recent years, and hence, further improvements in this area are desired.